Refactor Package structure

This commit cleans up the package structure, to make two new top-level
packages `internal` (for internal-use packages) and `pkg` (for general
shared utility code).

pkg/execx/look.go

@@ -0,0 +1,15 @@
+package execx
+
+import (
+	"os/exec"
+	"path/filepath"
+)
+
+// LookPathAbs is like [exec.LookPath], but always returns an absolute path
+func LookPathAbs(file string) (string, error) {
+	path, err := exec.LookPath(file)
+	if err != nil {
+		return "", err
+	}
+	return filepath.Abs(path)
+}